Solving life-and-death problems would be perfect.
Leave the ability to change but restrict it to the most difficult puzzle they solved. Maybe they have to solve 10 in a row at a particular level to get the right to choose that rank. 10 in a row for life-and-death is not outside of the ordinary. Five in a row would be charitable.
If they solve all the problems, the ranks change locks so they cannot sandbag. And, of course, is provisional to create the extra layer of integrity without requiring human resources.
Make changing ranks a serious issue and people will treat it seriously.
They can always grind to the top. Nobody is stopping that.
It will require a well developed life-and-death problems section. People will have to be tasked with double checking that user submitted problems are valid in their solution, categories and rank.
I’ve discussed this here: Suggestion: “Interactive” Go